This invention relates to medical ultrasonic imaging, and in particular to systems for improved imaging of contrast agent included in imaged tissue.
The nonlinear response of contrast agents such as microbubbles improves the detectability of microbubbles in tissue. Contrast agents effectively generate unique ultrasound signals such as second harmonic, third harmonic, subharmonic, or other harmonics that are not transmitted in significant quantities by a transducer. Good second harmonic contrast agents, in particular, have been shown to generate strong signals with minimal destruction of a population of contrast microbubbles.
Another unique property of contrast agents is that they are disrupted by impinging sound waves. Ultrasound pulses in particular can move, crack, rearrange, split, and destroy microbubbles and their gas-encapsulating shells. When this type of disruption occurs, at least two received pulses will be decorrelated when at least two identical pulses are transmitted during at least two separate transmit events. A loss-of-correlation (LOC) between received signals may be used to detect contrast agents, since areas that contain little or no contrast agent will return similar signals among the multiple received pulses and produce strong signal correlations.
There is a need for an improved LOC imaging technique that offers excellent specificity for contrast agents, high bandwidth resolution, and high frame rates. Further, there is a need to increase the spatial uniformity of detected contrast agents within an image frame when LOC techniques are implemented. Still further, there is a need to offer a contrast-only image that may or may not be displayed with an image substantially free of detected contrast agent.
Among the prior-art techniques for detecting contrast agents at least five disadvantages exist, as follows.
1.) Poor Axial Resolution Due to Narrowband Receive Filtering
A technique often labeled as xe2x80x9charmonic imagingxe2x80x9d specifically filters each received pulse/signal with the goal of suppressing fundamental energy and retaining second harmonic energy. This technique limits the bandwidth of returned signals and therefore limits axial resolution. xe2x80x9cPower Harmonicsxe2x80x9d by ATL or harmonic xe2x80x9cPower Contrast Imagingxe2x80x9d (PCI) by Acuson are two examples of this technique. Since the ratio of returned contrast agent energy to tissue energy for second harmonic signals is typically larger than the ratio of returned contrast agent energy to tissue energy for fundamental signals, second harmonic signals have been preferred for imaging contrast agents. By adequately suppressing fundamental energy by filtering each received pulse separately within a set of multiple received pulses, less tissue flash is introduced into the images if a clutter filter is applied across the two or more received pulses at each range. The purpose of the clutter filter is to remove signals from stationary or slowly moving targets. Since returned fundamental signals are the largest amplitude signals returned from stationary or slowly moving tissue as compared to harmonic signals, separately pre-filtering each returned pulse with a narrowband filter that suppresses fundamental energy reduces the amount of stopband rejection and/or stopband width required of the clutter filter. Fewer taps may be used with these types of clutter filters, and therefore fewer pulses need to be transmitted, thereby improving frame rates and minimizing unnecessary agent destruction.
Other examples of prior art that selectively filters second harmonic signals include xe2x80x9cHarmonic Power Mode Doppler . . . xe2x80x9d by Burns et. al [1] and xe2x80x9cUltrasonic Diagnostic Imaging with Contrast Agentsxe2x80x9d by Averkiou [2]. The former reference specifically describes a narrow-band digital filter in the Instrumentation subsection of the Methods section. The latter specifically describes the desire for narrowband filtering in Col. 5, lines 23 through 35, with reference to FIG. 6, and Col. 6, lines 13-19. Full citations for these and the other references cited in this section are provided in Table 1.
In other prior-art systems that selectively image the fundamental signal and suppress the second harmonic signal, e.g., the fundamental PCI imaging mode offered by Acuson, axial resolution is similar or inferior to those techniques that specifically suppress fundamental frequency components and retain second harmonic frequency components.
2.) Inability to Separate an Anatomical Reference Image from a Contrast-Only Image
The two-pulse, alternate polarity techniques (U.S. Pat. Nos. 5,706,819 [3], 5,951,478 [4], and 5,632,277 [5]) and multiple-pulse alternate polarity techniques (xe2x80x9cPulse Inversion Dopplerxe2x80x9d U.S. Pat. No. 6,095,980 [6]) and some Contrast Pulse Sequences (Reference [7]) are unable to show an accurate second harmonic contrast-only image, since tissue second harmonic signals cannot be accurately separated from contrast agent second harmonic signals. Techniques that alternate the transmit envelope polarity between different transmit events modulate the fundamental frequency signal components into the stopband of a clutter filter, which is applied across multiple received pulses at a single point in space, while second harmonic frequency signal components remain unmodulated and minimally suppressed by the passband of the clutter filter. Since the second harmonic signals generated from nonlinear propagation through tissue are not adequately suppressed, a stationary or slowly moving tissue image is inherently integrated with detected contrast agent signals. This is most significant for high mechanical index (MI) imaging, where tissue harmonic signals are strong. This is a significant clinical limitation, since contrast-only images can look much different than images that integrate contrast agent information with anatomical information.
Techniques that can generate contrast-only images can offer additional diagnostic information not available from techniques that alternate the transmit envelope polarity between different transmit events. For example, a 1 cm diameter lesion as seen in a B-mode image may appear as a 0.5 cm diameter lesion in a contrast-only image due to strongly anechoic areas near the perimeter of the lesion absorbing contrast agents. Without the ability to separate these two types of images after the acquisition and detection of a single frame, diagnostic confidence is reduced. Contrast-only image information can facilitate more accurate diagnosis and provide functional information.
3.) Inability to Pre-Scan a Region Before Imaging with a Contrast-Specific Imaging Technique
Current techniques used for contrast agent imaging such as the two-pulse techniques mentioned above lack the ability to pre-scan a region at low power before generating an image showing detected contrast agent with high specificity. For some contrast agents, such as Levovist, high MI scanning is preferred for achieving high agent specificity. However, high MI scanning disrupts the contrast agent, and can quickly deplete a region of contrast agent. Imaging techniques that inherently integrate anatomical tissue signals with contrast-specific signals do not facilitate efficient acquisition and user-selectable display of high-specificity contrast agent images. Once the transducer is placed on the subject, contrast agent is immediately disrupted and depleted. Pre-scanning with an alternating polarity pulsing technique at low transmit power to locate an area of interest and then manually turning up the transmit amplitude/power can be used, but contrast agent is destroyed before the optimal maximum transmit power is reached. Thus, unnecessary agent destruction occurs before the high specificity contrast agent image is generated. Further, optimal specificity may not be possible due to the lesser contrast agent concentration. A manually-activated, instantaneous, substantial increase in transmit power would be useful, but the inability to separate the background image from the contrast-only image still makes this type of technique suboptimal. The current procedure used with the alternating-polarity pulsing techniques is to begin a sweep through an organ, such as the liver, and hope to capture a slice, or slices, of interest through the organ. There often exists the possibility that a small lesion may have been missed, reducing diagnostic confidence, and demanding the need for further scanning and possibly further contrast agent injections.
4.) Insufficient Agent Specificity and Poor Sensitivity to Slow Flow or Small Variations Between Received Pulses from Separate Transmit Events
Incoherent LOC techniques have been disclosed that detect the presence of contrast agents within each received pulse and then subtract the two resulting detected signals. These phase-insensitive techniques can be less susceptible to color flash, or tissue flash, due to movement between transmitted pulses, but these techniques lack the sensitivity to small variations between received pulses. This reduces specificity and sensitivity. Examples of such incoherent techniques are disclosed in references [8] and [9].
5.) Poor image uniformity throughout an image displaying detected contrast agent
Conventional contrast agent imaging utilizes a focused transmit beam that generates a dominant focus with a specific focal region of high peak pressure. A single focus during a single transmit event generates excellent lateral resolution at or near the focal region, but fails to distribute peak pressures uniformly over a large depth-of-field, or section of ranges. Small transmit apertures can increase the depth-of-field uniformity, but at the expense of lesser transmit pressures in the field containing the contrast agents and degraded lateral resolution. A technique disclosed in reference [2] describes a method of stitching together received signals from multiple transmit events after a transmit focus has been moved between each transmit event. The resultant signal for a specific zone is most heavily weighted by the transmit event that sets the transmit focus within the specific zone of interest, as discussed at col. 9, line 56 through col. 11, line 29 of reference [2]. This technique may provide increased image uniformity, but at the expense of frame rate, the possibility of misregistration between separate transmit events, and unnecessary agent destruction.
A need presently exists for an improved ultrasonic imaging method and apparatus (1) that provide improved axial resolution when detecting contrast agent, (2) that generate contrast-only images, background or anatomical reference images, and images that combine both contrast agent information and background or anatomical reference information efficiently and reliably, without requiring rescanning of the tissue. Preferably, this would be done while minimizing the undesired destruction of contrast agent during orientation scans conducted prior to the contrast agent imaging. Also, such a method and apparatus preferably offer excellent specificity for a contrast agent while maintaining acceptable tissue suppression, and they preferably function reliably over a large depth of field without unnecessary contrast agent destruction, significant image artifacts, or excessively degraded frame rate.
The systems described below produce images of contrast agents with diagnostic sensitivity, high specificity, excellent detail resolution, improved uniformity of detection, high frame rates, and the ability to separately display three types of images: contrast-only images, images that contain anatomical information only, and images that integrate anatomical information with detected contrast agent information. These systems further include a single user-interface switch that allows a clinician to quickly switch between the three different types of images during a contrast examination or after images of an examination have been stored.